Typically, an integrated pump and energy recovery system is used to pump an input stream of fluid for purification through a membrane or filter, such as a reverse osmosis membrane, at a higher pressure. A stream of brine or other concentrated unpurified material is then discharged under pressure from such a membrane or filter. Typically energy is recovered from the discharge stream still under pressure, and then such recovered energy is used for a useful purpose, for example, to reduce the amount of energy that the pump would otherwise have to expend in order to pump the input stream of fluid into the system, thereby making operation of the purification system more efficient.
When a semi-permeable membrane divides two fluids of different salinities, osmosis occurs. To achieve equilibrium of the chemical potential across the membrane, liquid flows through the membrane into the more concentrated solution. This flow continues until concentrations on either side of the membrane are equal, unless the osmotic pressure is reached. Reverse osmosis (RO) is a membrane-technology filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of the selective membrane. The result is that the solute is retained on a pressurized side of the membrane and the pure solvent is allowed to pass to the other side of the membrane.
Efficiency of the reverse osmosis process may be improved by recovering energy from the high pressure waste brine. Known methods of pumping and of energy recovery include, for example, some combination of: plunger pumps with belt drives and pulsation dampeners, centrifugal pumps, sumps and sump pumps, reverse flow pump and Pelton wheel energy recovery turbines, hydraulic turbo chargers, flow work exchangers, and variable frequency drives.
In an integrated pump and energy recovery system using a plurality of piston-cylinder arrangements and valves for pumping the fluid medium, there are issues associated with pressure drops and loss of energy efficiency. Energy loss due to pressure drop is affected by the flow rate of fluids through valves which in turn is dependent on the velocity profile of the cylinders' pistons. The velocity profile also has an effect on life of seals that abrade against moving pistons.